Regulations governing emission limits for combustion engines, particularly in motor vehicles, are becoming increasingly stringent. Among the methods used to satisfy these requirements, catalytic converters are installed for after-treatment of exhaust gases by selective catalytic reduction (SCR) of the nitrogen oxides (NOx) contained in the gases. Ammonia must also be introduced so that the reaction can take place in the catalytic converter. This is assured by controlled injection of an aqueous solution of urea from a reservoir in the vehicle. Environmentally harmful nitrogen oxides are almost entirely converted into steam and atmospheric nitrogen. The industry has given this urea solution the name AdBlue®, or DIESEL EXHAUST FLUID (DEF) in the United States. Since the urea solution is susceptible to gelling and even freezing at low temperatures, the known liquid containers provide for the integration of heating elements inside the liquid container. This also enables urea solution that has already frozen to be thawed, for example when cold starting.
A heatable plastic tank in which a heat radiating heating coil is located inside the tank to heat the liquid in the container is known from EP 1 640 577 B1. The heating coil disposed in the base area of the container protrudes from the tank in two places, where it is connected permanently to two insert elements, these being encased in the plastic of the container wall. The heating coil is also supported in the base area. For this purpose, an area of the inner wall of the tank serves as a bracing element, and the heating coil is permanently enclosed in the plastic of the tank wall in the support area by sintering.
A similar bracing arrangement is also disclosed in DE 102 56 727 A1, which also describes a container for holding urea solutions with a heating element for controlling the temperature of a urea solution, in which the heating element is arranged in the base area of the container to make better use of the rising heat and is attached by fixing elements that are welded to the inner wall of the container. The fixing elements hold the heating element in such manner that it remains in a fixed position even when subjected to the strong vibrations caused by uneven road surfaces, such as occur when motor vehicles are operated regularly. Among various suggestions for preventing corrosion, the fixing elements may be made from an aluminium material and welded to the heating coil.
However, one disadvantage of such bracing and attachment devices for the heating coil inside the plastic tank is that it is impossible to prevent thermal expansion of the material as a result of the heat experienced during subsequent operation. This particularly affects the heating element, which is susceptible to expansion when warming up. Yet the same consideration also applies to the liquid container itself, since changes in length can occur here too, for example due to shrinkage following demoulding, or as a result of thermal expansion induced by thermal input. Material deformations in the known liquid containers may lead to relative movement between the bracing area and the connecting area for the connection ends of the heating coil. Since the attachment between the heating element and the liquid container is fixed, this can create stresses in the liquid container wall. In addition, particularly vehicles are not operated all the time, and temperature fluctuations may occur even during operation, and repeated heating and cooling cycles cause a kind of alternating stress. This subjects the container wall to dynamic loads that are constantly deforming the component. Depending on the material or construction of the container, this may result in cracks being formed, particularly at the critical fixing points, which in turn entails the risk of leaking.
It is further known to support the heating element on metal retaining clips at least portions of which have been encased in the plastic inside the container by sintering. In this case, however, liquid gets into the space between the metal retaining clips and the plastic wall by capillary action, which may possibly result in seepage leaks through which the liquid may escape from the container.